This invention relates generally to a windshield antenna, for a vehicle and, more particularly, to an antenna which is mounted on a rear windshield of a vehicle on which a rear window defogger (defroster) comprising heating wires is mounted.
FIG. 1 shows one conventional antenna of the type described. More specifically, a plurality of heating wires 2 which together constitute a window defogger are mounted on a rear windshield 1 of a vehicle in parallel, closely spaced relation to one another, and extend longitudinally of the rear windshield 1. A pair of first and second antenna elements 3 and 4 are mounted on the rear windshield 1 parallel to and above and below the defogger heating wires 2. Electric power is supplied from a battery 6 of the vehicle to the defogger heating wires 2 via a power supply wire 8a, which is wound on a ferrite core 7. A grounding wire 8b also is wound on the ferrite core 7 so that the power supply wire 8a, and hence the defogger heating wires 2 are grounded to the body of the vehicle. Thus, this arrangement is set so that the inductance produced by the wires wound around the ferrite core 7 has a high impedance at a receiving frequency, so that the defogger heating wires 2 are isolated from the earth of the vehicle body, thereby reducing the influence of the defogger wires 2 in the directivity of the antenna elements. Moreover, the power supply wire 8a and the grounding wire 8b are connected together electrically through the ferrite core 7, and this prevents noise from developing in the defogger heating wires 2. Reference numeral 5 denotes rear pillars on the vehicle.
The first antenna element 3 serves as an antenna for receiving both FM (frequency-modulated) waves and AM (amplitude-modulated) waves, while the second antenna element 4 serves as a sub-antenna for FM waves. The outputs of the two antenna elements 3 and 4 are connected respectively to amplifiers 9 and 10 contained in the rear pillar 5. Particularly when receiving FM broadcasting, one of the first and second antenna elements 3 and 4 having a better signal reception condition is selectively used; such is known as "diversity reception" in the trade.
In the above conventional windshield antenna, since the upper side and lower side of the rear window glass 1 are different in length from each other, it is difficult to design the two antenna elements 3 and 4 to have the same reception characteristics. The two antenna elements 3 and 4 themselves differ in gain because of the differences in size and shape. Therefore, when the diversity reception is performed by the antenna elements 3 and 4, a satisfactory diversity effect cannot be obtained, since the two antenna elements 3 and 4 have different gains in the first place.
Further, it is difficult to design the FM antenna elements 3 and 4, disposed respectively on the upper and lower sides of the defogger heating wires 2, to have the same reception characteristics because of the difference in their spatial positions from the surface of the ground, the difference between the upper and lower their antenna patterns 3 and 4 in antenna pattern such as the shape and size in view of the receiving band, and so on. Therefore, a difference in gain occurs between the two antenna elements. As a result, when diversity reception is performed by the antenna elements 3 and 4, the rate of selecting one of the two antennas which has a higher gain is high because of the presence of such gain difference. This prevents a good diversity reception from being performed.
Still, further, since the first antenna element 3 is designed to receive both FM and AM waves, it is quite difficult to obtain good reception characteristics with respect to both. Further, the amplifiers 9 and 10 have to be mounted within one of the rear pillars 5 because of the position of mounting of the antenna elements 3 and 4. This considerably limits the size and shape of these amplifiers.